Wait I'm confused if the one he is in love with is just the goddess in disguise then what's going on with the shrine Maiden? Did the goddess use the shrine maidens form as her disguise?
I think the implication is that since the goddess is such a greedy, lazy, half-assing dirtbag (in a rush to rip off MC, always loafing about at Starbucks) she's also exactly the kind of person who would just use the first person she saw at the shrine as the model for her disguise.
I think the shrine maiden is just a hapless bystander here, who's never had any interaction with the MC at all. The MC's "love" is such a shallow crush that he's never spoken to the Starbucks girl and doesn't even know her name. Now the goddess is selfishly going to try and push them together.
Also, the stamp that appears on the goddess' forehead is the 売約済 from the 売約済み or
baiyakuzumi in the series' title. Here it basically just means "SOLD," like she just took herself off the matchmaking market. Common in retail signage for "product sold out" or "this one's already sold/reserved."